Prime Minister Theresa May said Britain has concluded it is “highly likely” that Moscow was responsible for poisoning a former Russian spy and his daughter with a military-grade nerve agent, calling the attack “indiscriminate and reckless.”

“This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals,” she said Monday in her address to lawmakers in Parliament. “It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk, and we will not tolerate such a brazen attempt to murder innocent civilians on our soil,” she added.

May said Britain will give the Russian ambassador until Tuesday to explain whether it “was a direct act by the Russian state” or the result of it having “lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others.”

She said the substance was identified as a nerve agent known as “Novichok,” developed by Russia.

“On Wednesday, we will consider in detail the response from the Russian state. Should there be no credible response, we will conclude this action amounts to an unlawful use of force by the Russian state against the United Kingdom and I will come back to this house and set out the full range of measures that we will take in response,” she said.

Earlier Monday, May called a meeting of the National Security Council to go over evidence of the poison attack.

She is under intense pressure to retaliate against Russia with tough sanctions and expelling diplomats, as well as other measures.

Skripal had been living in England since being freed as part of a 2010 US-Russian swap for sultry spy Anna Chapman, who had been living undercover in New York City.

He had been convicted in Russia of spying for the British and sentenced to 13 years in prison in 2006.

Yulia was visiting from Moscow.

In the past six years, Skripal’s wife, older brother and son have died.

Wife Liudmila died in 2012, and a certificate recorded the 59-year-old’s death as “disseminated endometrial carcinoma,” but media reports at the time said she died in a car crash.

Then in the past two years, Skripal’s older brother died in Russia and his 43-year-old son died while visiting St. Petersburg, Russia, with his girlfriend. He had been taken to the hospital for liver failure.

The Skripals’ case raised similarities to the death of former Russian spy Alexander Litvinenko, who died of radioactive poisoning in a London hospital in November 2006.

An examination of his death said there was a “strong probability” he was killed on orders from the Russian secret service.

Dmitry Peskov, a spokesman for Russian President Vladimir Putin, said Skripal was poisoned in Britain, so the incident has “nothing to do with Russia, let alone the Russian leadership.” Peskov also said the Kremlin hasn’t heard any official statements of Russian involvement.